In the visual light receptor cell, the movement of ion across its membrane and the control of this ion flow by light-released intracellular excitatory transmitter have been shown by us to be basic in the initiation of the light detection process in the visual system. The nature of this excitatory transmitter, how light and the visual cell control its passage across membranes, and how this transmitter in turn regulates movement of ion across the visual cell membrane are the focal points of our investigations.